1. Field of the Invention
The invention relates to a novel process for preparing dielectric coatings by the plasma polymerization of a monomer or a monomer and diluent. More particularly, the invention relates to a novel process for preparing dielectric coatings by plasma polymerization whereby the dielectric constant of the polymer coating prepared from a monomer or monomer and diluent can be varied and controlled.
2. Description of the Prior Art
Void-free organic insulators are commonly used for electrical insulating applications, such as capacitors, electrical standoffs and free-throughs. The major problem encountered with such insulators is electrical degradation over time which is often caused by corona at the interface between the insulator and the surrounding gas or liquid. The corona, which is driven by the large electric field at the surface, enhances the decomposition of the organic insulator. The large surface electric field is provided by the large discontinuous change in the dielectric constant across the insulator-gas (or liquid) interface. Organic insulators have large dielectric constants ranging in values from about 3 to 6, while gases have values near 1 and liquids have values in the range of about 2.5 to 6.5. A typical example of the surface electric field across a boundary with a discontinuous change in the dielectric constant is illustrated in FIGS. 1a and 1b. To avoid decomposition and ultimate electric failure of the insulator a reduction in the operating electric field at the surface is required. If the insulator were coated with an organic dielectric film which would eliminate the discontinuous change in the dielectric constant, decomposition of the insulator could be retarded.
This organic dielectric coatings have been produced using electron beam polymerization, photopolymerization and plasma polymerization processes. The measured dielectric constants of the coatings made according to these prior art processes vary with the initial monomer employed but appear to be independent of the polymerization process and deposition conditions used. The following U.S. Patents disclose typical prior art plasma polymerization processes for producing thin organic dielectric coatings: U.S. Pat. Nos. 3,069,283 (Coleman), 3,252,830 (Cummin et al.), and 3,518,108 (Heiss, Jr. et al.). These prior art processes can be used to produce dielectric coatings having a wide range of dielectric constants when using different gases but each gas or gas mixture gives a single value for the dielectric constant. For example, values of the dielectric constants of the dielectric coatings prepared according to the plasma polymerization process disclosed in the Heiss, Jr. et al. patent were found to be in the range of from 2.66 to 5.1.
If an electrical insulator were coated using a prior art polymerization process to reduce corona at its surface, the discontinuous change in the dielectric constant would not be eliminated. Decomposition of the insulator would still occur but at a reduced rate. A reduction in the operating electric field would be required in order to minimize decomposition of the insulator. The electrical characteristics of a coating using a prior art processes are illustrated in FIGS. 2a and 2b.